Chapter 3 Synthesis of soft nanoparticles based on block copolymer self-
3.5 Conclusions
The synthesis of polymeric nanoparticles was successfully achieved by cross-linking of PB-containing block copolymers self-assembled into spherical micelles. Cross-linking of the micelles in solution did not alter their spherical structure and narrowly distributed nanoparticles were obtained. The size of the nanoparticles can be tuned by the molecular weight of the block copolymer and depends also on the nature of the solvent used. Self-assembly of B-M block copolymer into micelles occurs in many different selective solvents but acetonitrile proved to be the best for spherical micelle formation, regardless of the composition and molecular weight of the block copolymers. Their micellar behavior is similar to those for strongly segregated block copolymers described by Förster and Antonietti. Upon cross-linking, the B-M nanoparticles loose their low glass transition temperature whereas B-nBMA and B-nBA nanoparticles still exhibit relatively low glass transition temperature after cross-linking reaction. Those latter might provide better impact toughness than B-M nanoparticles when introduced in a stiffer material, provided they are dispersed in a matrix which is compatible with the shell of the nanoparticles (PMMA, PnBA or PnBMA).
Water-soluble nanoparticles could also be successfully obtained from B-tBMA cross-linked micelles. The hydrolysis of the tBMA corona of the cross-linked nanoparticles led to water soluble B-MAA nanospheres. Their glass transition temperature was also strongly shifted to temperatures above room temperature. They can be used as nanomodifiers for waterborne PU coatings.
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Chapter 4 Hyperstars
Chapter 4
Synthesis of hyperbranched block copolymers (Hyperstars) based on Polybutadiene
4.1 Introduction
Branched polymers have become a field of considerable scientific interests because of their particular properties differing from their linear analogs of similar molecular weights, in both solution and bulk. They generally present lower viscosities, are highly functionalizable and their solubility in solvents is higher where they usually behave as molecular micelles (globular structure)1. Hyperbranched polymers, in contrast to dendrimers, are complemented by an ease of synthesis which does not require tedious sequential step synthesis. Such simplification in synthesizing hyperbranched polymers allows them to be produced on a large scale and to be involved in application demanding significant amount of material.
Despite the numerous existing techniques to synthesize hyperbranched polymers including cationic, anionic, group transfer, controlled radical and ring-opening polymerization2, 3, a straightforward methodology for branched polymers based on diene monomers has not been developed yet. Recently, Frey et al. demonstrated a two-step synthesis of branched polymers based on isoprene, so-called “Ugly-Stars”4, by condensing preformed polymer segments with each other. As another alternative to classic AB2 or A2 + B3 polycondensation, Fréchet et al. brought up the “self-condensing vinyl polymerization”
(SCVP)5 that later gave rise to its anionic equivalent (ASCVP) mainly investigated by Baskaran et al.6, 7 on styrenic monomers like divinylbenzene (DVB) and 1,3-diisopropenylbenzene. The principle relies on the in-situ creation of a species bearing simultaneously an initiating site (B*) and a polymerizable group (A) so-called “inimer” (initiator-monomer) AB*. The asterisk indicates an active site.
Within our group, Nosov et al. reported a method for the synthesis of highly branched polybutadienes via anionic self-condensing vinyl copolymerization (ASCVCP) of a DVB based inimer and butadiene8.
Herein we present a method for the synthesis of two-component highly branched polymers. The hyperbranched core is first synthesized using the method developed by Nosov et al., i.e. anionic self-condensing vinyl copolymerization (ASCVCP) of a DVB based inimer and butadiene. This reaction is followed by the subsequent anionic polymerization of a poly((meth)acrylate) corona. The resulting polymer is a hyperstar with a hyperbranched polybutadiene core protected by a poly((meth)acrylate) corona. Different isomers of DVB were used for this purpose.
4.2 Experimental part
4.2.1 Materials
Sec-butyl lithium (sec-BuLi) (Aldrich), dibutylmagnesium (Bu2Mg), triethylaluminum (Et3Al) (Aldrich), iso-butyl aluminum (2,6-di-tert-butyl-4-methylphenolate)2 (iBuAl(BHT)2) (0.45 mol/L in toluene, Kuraray Co. Ltd.) were used without further purification. 1,3-Butadiene (BD) (Messer Griesheim) was passed through columns filled with molecular sieves (4Å) and basic aluminum oxide and stored over Bu2Mg. Methyl methacrylate (MMA), n-butyl (meth)acrylate (n-B(M)A) (BASF) were condensed from Et3Al on a vacuum line and stored at liquid nitrogen temperature until use. Toluene (Merck) was distilled from CaH2 and potassium. 1,2-Dimethoxyethane (DME) and tert-butylmethyl ether (TBME) were purified using a certain amount of sec-BuLi and condensed on a vacuum line.
4.2.2 Synthesis of Divinylbenzene (DVB) from its corresponding aldehyde
Para- and meta-DVB (p-DVB, m-DVB) were synthesized according to the literature9 from their corresponding dialdehydes, terephtalic aldehyde and isophtalic aldehyde (Aldrich), by a Wittig olefination reaction. Typically, 0.16 mol (56 g) of triphenylmethyl phosphonium bromide, 0.2 mol (28 g) of K2CO3 in 120 ml of dioxane and 1.8 ml of distilled water were introduced into a round bottom flask equipped with a condenser and a magnetic stirrer.
After dissolution of 0.08 mol (10.8 g) of the aldehyde in 40 ml of dioxane and 0.6 ml of
Chapter 4 Hyperstars distilled water, it was successively introduced into the reaction vessel. The reaction mixture was refluxed for at least 12 hours. After reaction, the inorganic salts were first filtered off and the solvent evaporated under vacuum. The resulted product was re-heated until liquid and added in hexane under vigorous stirring. The triphenylphosphine oxide precipitated and was filtered off and washed with hexane. Hexane was then evaporated under vacuum and the resulting yellowish product subjected to flash chromatography on SiO2 gel.
p-DVB, m-DVB and technical DVB (T-DVB) (Aldrich) were condensed on a vacuum line from Bu2Mg and kept at liquid nitrogen temperature until use.
4.2.3 Anionic Self-Condensing Vinyl Copolymerization (ASCVCP) of (p-, m-, T-) DVB and butadiene (BD) yielding hyperbranched core precursor
All polymerizations were carried out under inert atmosphere in a thermostated glass reactor (Büchi, Switzerland). Typically, to 200 ml of toluene was added 0.15 mol (18.3 ml) of TBME as polar additive to control the microstructure. The reactor was cooled down to 0 °C.
3.8 mmol (2.74 ml) of sec-BuLi and 3.8 mmol (0.55 ml) of DVB were introduced, in this order, with a syringe into the reaction vessel and an immediate deep red color appeared, sign of the rapid formation of the inimer. 0.13 mol (10.1 ml) of butadiene were condensed from Bu2Mg into an ampoule cooled down to -20 °C and then added to the reaction mixture.
The ASCVCP of DVB and butadiene was left to proceed for 24 hours at 0 °C. The reaction was terminated with degassed methanol. The solvent was evaporated, the product dissolved in hexane and subjected to flash chromatography over SiO2. The molecular weights and molecular weight distributions of the DVB-BD hyperbranched copolymer were measured by GPC.
4.2.4 Synthesis of (p-, m-, T-)DVB-BD-PMMA hyperstar
After the ASCVCP of DVB and butadiene (DVB-BD), an aliquot was withdrawn for characterization. A mixture of 0.11 mol (12.3 ml) DME and 0.02 mol (51.2 ml) iBuAl(BHT)2
was introduced into the reactor to enable the subsequent polymerization of 0.1 mol (10.6 ml) methyl methacrylate (MMA), in a controllable manner at room temperature. After adding MMA, the reaction solution turned yellow. This color is characteristic of the complex formed between the aluminum compound and the (meth)acrylate monomer10. When the
reaction is complete, no complexes are formed anymore and the reaction medium is colorless. Thus, the end of the reaction was visually remarkable when the yellow color of the solution vanished. The reaction was terminated with degassed methanol and the reaction mixture was stirred for an hour with an aqueous solution of sulfuric acid (2 %wt) to remove the aluminum additive. The organic phase was extracted and washed with distilled water.
The polymer was finally precipitated in methanol and dried under vacuum at room temperature. After dissolution in hexane, it was subjected to a silica gel column. The molecular weights and molecular weight distributions of the DVB-PB-PMMA branched block copolymers were measured using GPC.
4.2.5 Synthesis of p-DVB-BD-PnBA hyperstar
After the ASCVCP of p-DVB and BD, an aliquot of the polymer was withdrawn for characterization and the reactor was cooled down to -20 °C. The mixture of DME (0.11 mol, 12.3 ml) and iBuAl(BHT)2 (0.02 mol, 51.2 ml) was added and 0.08 mol (11.1 ml) of the monomer nBA was introduced with a syringe drop-wisely. The reaction was terminated with degassed methanol and the reaction mixture was stirred for an hour with an aqueous solution of sulfuric acid (2 %wt). The organic phase was extracted and washed with distilled water. The polymer was finally precipitated in methanol and dried under vacuum at room temperature. After dissolution in hexane, it was subjected to a silica gel column. The molecular weights and molecular weight distributions of the p-DVB-PB-PnBA branched block copolymers were measured using GPC.
4.2.6 Synthesis of p-DVB-BD-PnBMA hyperstar
After the ASCVCP of p-DVB and BD, an aliquot of the polymer was withdrawn for characterization and a mixture of DME (0.11 mol, 12.3 ml) and iBuAl(BHT)2 (0.02, 51.2 ml) was added. 0.07 mol (11.1 ml) of the monomer nBMA was introduced with an ampoule into the reactor which was warmed up to room temperature. The reaction was terminated with degassed methanol and the reaction mixture was stirred for an hour with an aqueous solution of sulfuric acid (2 %wt). The organic phase was extracted and washed with distilled water. The polymer was finally precipitated in methanol and dried under vacuum at room temperature. After dissolution in hexane, it was subjected to a silica gel column. The
Chapter 4 Hyperstars molecular weights and molecular weight distributions of the p-DVB-PB-PnBMA branched block copolymers were measured using GPC.
4.3 Results and discussion
4.3.1 Anionic Self-Condensing Vinyl CoPolymerization (ASCVCP) of DVB-BD
The polymerization is depicted in Scheme 1. Three different isomers of the inimer were used: p-DVB, m-DVB and technical DVB (T-DVB), commercially available which is a mixture of p-DVB, m-DVB and 35 % of ethylstyrene. For all polymerizations, the following conditions were used: MTBE/Li = 40/1, BD/DVB = 32/1 at 0 °C for 24 hours. In the case of T-DVB, as it contains only 65 % of DVB, the ratio BD/DVB was recalculated to be 50/1.
Scheme 1. Synthesis of hyperbranched DVB-BD via ASCVCP
The reactivity of DVB can be assimilated to the reactivity of styrene. It is known that in hydrocarbon solvents, the reactivity ratios of styrene and butadiene favor the formation of so-called “tapered” block copolymers. To avoid this, TBME was added as a “randomizer” so that the reactivity ratios of DVB and butadiene become closer to each other resulting in the formation of a random copolymer.
The hyperbranched samples are denoted p-DVB-BD, m-DVB-BD and T-DVB-BD according to the type of isomer used. 1H NMR was measured for the three different isomers and spectra are displayed in Figure 1. For each of them, the presence of an aromatic signal at around 7 ppm, confirmed the presence of DVB in all polymers. The content of 1,4 units of PB was also calculated according to the vinyl signals at 4.9 and 5.4 ppm and was found to vary between 38 and 43% due to the presence of polar additive (TBME). Molecular weights and
molecular weight distributions are reported in Table 1. GPC using linear PB standards calibration and GPC using MALS detector were both measured.
Figure 1. 1H NMR spectra (300 MHz) of p-, m-, T-DVB-BD hyperbranched copolymers in CDCl3.
Table 1. Molecular parameters for ASCVCP of different DVB isomers with 1,3-butadiene in toluene at 0 °C, MTBE/Li = 40, BD/DVB = 32 = γ.
10-3Mna
(g/mol) PDIa 10-3Mnb
(g/mol) PDIb αc
p-DVB-BD 5.2 3.2 8.2 3.1 0.45
m-DVB-BD 4.2 1.1 3.2 1.06 (0.33)
T-DVB-BD 5.3 1.6 4.8 1.2 0.59
aGPC, PB linear standards, bGPC/MALS detection, cMark-Houwink-Sakurada exponent, GPC/viscosity detection
The ASCVCP of BD and DVB was studied earlier in our group8 and possible routes for the reaction were proposed (see Scheme 2). In the case of the para- isomer, the rate constant k1
is significantly higher than the rate constant for the second addition, k2. This is due to the fact that both vinyl groups are conjugated to each other. More specifically, the first addition of sec-BuLi induces an extensive charge delocalization stabilizing the formed carbanion and therefore, deactivates the second vinyl group. The addition of comonomer M, in our case butadiene, will favor the macroinimers mechanism as kBM > kBA. This reactivity promotes the formation of macroinimers and their subsequent self-condensation to yield hyperbranched polymers.
Chapter 4 Hyperstars
Scheme 2. Synthetic strategies towards branched polybutadienes8. A*, B*, M* denote active sites, a, b, m reacted ones.
The reaction of p-DVB and BD was followed by GPC and data are shown in Figure 2 and Table 2. In the early stage of the polymerization, macroinimers of linear polybutadiene (A-b-M*) are formed as kBM > kBA. After 12 hours of reaction, self-condensation of the macroinimers can already be assessed by the presence of a tiny shoulder at lower elution volume. As the polymerization proceeds, more shoulders are appearing at lower elution volume. The concentration of macroinimers decreases all along the polymerization as self-condensation occurs and the amount of the branched products increases. The final polymer is therefore a mixture of macroinimers and their self-condensation products in various concentrations.
More details about the mechanism of the ASCVCP of BD with p-DVB as well as the effect of solvent, temperature and comonomer ratios are discussed by Nosov et al.8
Figure 2. GPC traces (RI signal) of p-DVB-BD after different reaction times. PB calibration.
Table 2. Molecular parameters at various polymerization time for the ASCVCP of p-DVB with 1,3-butadiene in toluene at 0 °C, MTBE/Li = 40, BD/DVB = 32 = γ.
Reaction time (hours) 10-3Mna
(g/mol) PDIa
12 2.6 1.2
24 2.9 1.4
45 3.4 1.6
99 3.7 1.7
aGPC, PB linear standard
GPC traces are displayed in Figure 3 for p-DVB-BD, T-DVB-BD and m-DVB-BD synthesized under the same conditions. The latter exhibits a monomodal narrow molecular weight distribution, indicating that mostly linear polymers are produced. Indeed, in the case of m-DVB, the rate constants k1 and k2 are of comparable values and both vinyl groups can, therefore, add sec-BuLi simultaneously forming a difunctional initiator instead of an inimer.
Molecular weights obtained from GPC with MALS detection confirmed this hypothesis. The theoretical length of a PB segment initiated from one site on the DVB is 32 repeating units.
When calculated, one DVB unit initiated by two equivalents of sec-BuLi presents a theoretical molecular weight of 3700 g/mol, i.e. 64 butadiene units. GPC gives a value of 3200 g/mol which means 56 butadiene repeating units. This last result is consistent with our explanation within the experimental errors.
Chapter 4 Hyperstars T-DVB is a mixture of meta- and para- isomers and some ethylstyrene. The hyperbranched copolymer T-DVB-BD shows a higher concentration in linear product than p-DVB-BD obtained from the para- isomer exclusively. This is probably due to the presence of ethylstyrene which cannot participate in the self-condensation reaction. The presence of m-DVB, as described earlier also mostly results in linear products.
Figure 3. GPC traces (RI signal) of DVB-BD copolymers with different DVB isomers. PB calibration (see Table 1).
When comparing molecular weights of the different hyperbranched products synthesized under the same conditions, p-DVB-BD reaches higher molecular weights than T-DVB-BD and m-DVB-BD. This can be related to the different mechanisms described earlier. In the case of the copolymerization with p-DVB, self-condensation occurs to yield hyperbranched polymers while m-DVB produces linear PB which does not seem to self-condense later on. T-DVB-BD can be described as a mixture of these two plus a certain amount of linear PB initiated from ethylstyrene. Therefore, its molecular weight is lower than that of p-DVB-BD but higher than that of m-DVB-BD.
The difference observed in the case of p-DVB-BD where molecular weights measured in GPC are lower than those measured in GPC/MALS also confirms that most of the polymer is branched. Indeed, branched polymers exhibit smaller hydrodynamic volume than their linear analogues and therefore lead to lower apparent molecular weights values. Mark-Houwink-Sakurada (MHS) parameters were also measured with GPC/viscocity. For the p- and T- isomers, α values, displayed in Table 1, are 0.45 and 0.59 respectively which is lower
than 0.74, the value for linear PB. This observation confirms further, the dense topology of the different polymers and therefore their branched structures. In Figure 4, Mark-Houwink-Sakurada plots are established for linear PB and hyperbranched p-DVB-BD. The contraction factors g’ = *ηbr+/*ηlin] were calculated from the plots and show that the density of the hyperbranched polymer increases with molecular weight (solid line in Figure 4). The extremely low α value obtained for m-DVB-BD (α = 0.33) is not reliable due to the very narrow distribution of the polymer.
Figure 4. Mark-Houwink-Sakurada plots for linear PB (■) and BD (□), contraction factors g’ of p-DVB-BD (solid line).
According to previous studies carried out by Nosov et al., higher molecular weights of the hyperbranched polymers can be obtained when increasing the comonomer ratio, γ, or the amount of polar additives (randomizer). Taking the exemple of p-DVB-BD, increasing the comonomer ratio strictly means increasing the amount of BD introduced into the reaction.
Therefore, macroinimers increase in molecular weight and through self-condensation the overall molecular weight of the hyperbranched copolymer is also increased. When the amount of polar additive is increased, the formation of macroinimers will, in the first place, occur faster and yield a high content of 1,2-PB microstructure. More importantly, increasing the amount of randomizer should result in kBM ~ kBA. Thus, DVB is distributed more randomly over the PB macroinimers increasing the number of potential branching points subsequently leading to higher molecular weights of the final hyperbranched polymer. At the same time, TBME facilitates the access to styrenyl anions and therefore promote
self-Chapter 4 Hyperstars condensation. GPC traces for p-DVB-BD synthesized with a high comonomer ratio, typically γ
= 32, and two different TBME/Li ratios are shown in Figure 5 and data are summarized in Table 3. Higher molecular weights are reached with TBME/Li = 40, the concentration in branched products is higher.
Figure 5. GPC traces RI signal) of p-DVB-BD with different TBME/Li ratios. PB calibration.
However, further increase in molecular weights is prevented by the intramolecular reaction occurring between B* (or M*) and A. This back-biting reaction consumes potential self-condensing sites limiting the degree of branching and subsequently the final molecular weight.
Table 3. Molecular parameters for p-DVB-BD hyperbranched copolymers synthesized with various TBME/Li
Table 3. Molecular parameters for p-DVB-BD hyperbranched copolymers synthesized with various TBME/Li